Our long-term objective is to determine the organization of cutaneous neurons of the dorsal horn, and their role in fine tactile discrimination. The somatotopic organization of the dorsal horn is essential to this role. We propose to study further the organization of cutaneous projections to the dorsal horn in order to test, refine or reject the presynaptic somatotopy hypothesis, which states that cutaneous afferents project only to those dorsal horn regions where their receptive fields lie within the receptive fields of dorsal horn cells. We have designed experiments to determine that aspects of the morphology of primary afferent projections to dorsal horn, and of dorsal horn cell dendritic trees, can be accounted for by the somatotopic organization of the region. These experiments will (a) test several specific predictions concerning single-axon projection patterns based on the presynaptic somatotopy hypothesis: and (b) correlate somatotopy and morphology of several identified types of primary afferents and dorsal horn cells. We will inject primary afferent axons in the white matter with HRP, followed by detailed mapping of the dorsal and ventral horns and Intermediate grey matter. The mapping will use extracellular multi-unit and single unit recordings, with reconstruction of all recording sites, as well as intracellular staining of as many spinal grey neurons as possible. Antidromic stimulation will be used to identify tract neurons. Computer-aided reconstruction will be used to allow us to model the formation of the monosynaptic components of dorsal horn cell receptive fields, and to predict the relative information-carrying ability of various ascending tracts in localization and two-point discrimination tasks. They will also be used in future studies of the development of dorsal horn somatotopy in neonatal animals, and reorganization following partial deafferentation.